The use of herbicides to control weeds or plants in crops has become almost a tmiversal practice. The relevant market exceeds a billion dollars annually. Despite this extensive use, weed control remains a significant and costly problem for farmers.
Effective use of herbicides requires sound management. For instance, time and method of application and stage of weed plant development are critical to getting good weed control with herbicides. Since various weed species are resistant to herbicides, the production of effective herbicides becomes increasingly important.
Unfortunately, herbicides that exhibit greater potency, broader weed spectrum and more rapid degradation in soil also have greater crop phytotoxicity. Crop hybrids or varieties resistant to the herbicides would allow for the use of the herbicides without attendant risk of damage to the crop. U.S. Pat. No. 4,761,373 to Anderson et al. is directed to plants resistant to various imidazolinone or sulfonamide herbicides. The resistance is conferred by an altered acetohydroxyacid synthase (AHAS) enzyme. U.S. Pat. No. 4,975,374 to Goodman et al. relates to plant cells and plants containing a gene encoding a mutant glutamine synthetase (GS) resistant to inhibition by herbicides that were known to inhibit GS, e.g. phosphinothricin and methione sulfoximine. U.S. Pat. No. 5,013,659 to Bedbrook et al. is directed to plants that express a mutant acetolactate synthase which renders the plants resistant to inhibition by sulfonylurea herbicides. U.S. Pat. No. 5,162,602 to Somers et al. discloses plants tolerant to inhibition by cyclohexanedione and aryloxyphenoxypropanoie add herbicides. The tolerance is conferred by an altered acetyl coenzyme A carboxylase(ACCase).
To genetically engineer plants for the purpose of herbicide resistance, the target of the herbicide first must be identified. This task can be very difficult. For example, the genome of E. coli is capable of expressing at least 60 amino acid biosynthetie enzymes. Plants are much more complex, and thus contain many more enzymes. The sheer number of potential targets, therefore, is a factor. Also, plant enzymes have proven difficult to purify, which hinders large-scale in vitro screening efforts. Further, the effect of a herbicide on a particular plant enzyme cannot necessarily be predicted on the basis of the effect of the herbicide on the microbial analogue. Various herbicides, e.g. aminotriazoles, affect different biosynthetie pathways in plants and microbes. See, Hilton et al., Arch. Biochem. Biophys. 112:544-547 (1965); Jeim and Larrinua, Plant Physiol. 91:1226-1231 (1989). Finally, plants have other resistance mechanisms such as rapid metabolism, and poor uptake and translocation of the herbicide which complicates elucidation of the enzyme target.